An exemplary fiber optic receiver 100 is illustrated in FIG. 1. The receiver 100 comprises a photosensitive input element 102 (e.g., a photodiode), a preamplifier (preamp 104), a postamplifier (postamp 106) generating a receiver output V1, and an optional one or more post processing circuits 108 generating a receiver output X.
In the receiver 100, the role of the preamp 104 is to convert currents (I) generated by the photosensitive input element 102 into corresponding voltages (V0). The postamp 106 then provides additional amplification to the voltage V0, so as to saturate its output voltage (V1) to a certain level.
In some cases, a receiver's postprocessing circuit(s) 108 may employ optical modulation amplitude (OMA) readback, which is a readback of the difference between a received optical signal's logic-one and logic-zero power levels. OMA readback can be useful as a real time digital diagnostic for determining the optical power of received signals. By way of example, in fiber channel and ethernet receivers, OMA readback can be used to detect broken or faulty optical fibers, or the lack of an input signal.
In receivers where OMA readback is a requirement, it is desirable that the receiver's preamp 104 be operated with a linear gain (i.e., a gain which causes its output to vary linearly with its input). However, the extent of a preamp's linear gain region is limited by at least three factors: postamp sensitivity, preamp supply voltage, and preamp gain magnitude. As shown in FIG. 2, the sensitivity 202 of the postamp 106, in combination with the magnitude of its corresponding preamp's gain, defines the lower end “A” of the preamp's linear gain region (i.e., “Linear Gain Region 1”). Similarly, the preamp's supply voltage 200, in combination with the magnitude of the preamp's gain, defines the upper end “B” of the preamp's linear gain region. Within these confines, the magnitude of the preamp's gain determines the extent (or width) of the preamp's linear gain region. Thus, if the gain of the preamp 104 increases from “Gain 1” to “Gain 2”, the linear gain region of the preamp 104 will shrink to that of “Linear Gain Region 2”.
As fiber optic technology progresses (e.g., as optical communication links get longer and optical signaling powers decrease), an optical receiver's preamp 104 needs to detect ever-smaller inputs and hence have a good signal-to-noise ratio and better sensitivity. At the same time, it remains desirable for the preamp 104 to have a wide linear gain region and acceptable overload performance (i.e., acceptable operation at higher optical powers). Unfortunately, better preamp sensitivity typically dictates a need for higher gain, while a wide linear gain region and better overload performance typically dictate a need for lower gain.
One way to provide a preamp 104 with better sensitivity, a wide linear gain region, and acceptable overload performance is to incorporate variable gain control circuitry into its receiver 100. Variable gain control circuitry operates to increase a preamp's gain as input power decreases, and then decrease the preamp's gain as input power increases, thereby providing the preamp 104 with better sensitivity at low input powers while preventing the preamp's output from saturating at high input powers. However, the use of variable gain control circuitry results in a variable relationship between a preamp's input (I) and output (V0), thereby making OMA readback difficult.